This invention relates to a solid state image sensing device, and more particularly a charge transfer device type image sensor.
Solid state image sensing device is well known and various improvements have been made. More particularly, so called buried channel type charge transfer device provided with a buried channel formed by diffusing into a semiconductor substrate an impurity having a conductivity type opposite to that of the substrate are disclosed in copending U.S. patent applications Ser. No. 296,507 filed on Oct. 10, 1972 and Ser. No. 403,244 filed on Oct. 3, 1973. While the buried channels can improve the efficiency of drifting electric carriers, there are the following two problems.
The first problem is that the construction of the drive circuit of the charge transfer device type image sensor is complicated. Thus, an image sensor should be provided with means for impressing a pulse to a shift electrode and a charge transfer electrode and means for impressing a pulse to a photoelectrode as indispensable elements. Accordingly, elimination of one of these pulse impressing means simplifies the construction of the drive circuit and decreases the chance of mis-operation.
The second problem is that as the quantity of the electric charge that can be accumulated in a potential well beneath the photoelectrode is relatively small, it is necessary to increase the area of respective picture elements beneath the photoelectrode in order to increase the charge quantity. As is well known in the art, the maximum charge quantity that can be accumulated in the buried channel layers beneath the photoelectrode and corresponding to respective picture elements can be approximately shown by (C.sub.1 +C.sub.2)V.sub.1, where C.sub.1 represents the effective electrostatic capacitance between a buried channel layer and a photoelectrode per one picture element, C.sub.2 the effective electrostatic capacitance between a buried channel layer and a semiconductor substrate per one picture element, and V.sub.1 the difference between the high and low level potentials impressed upon the photoelectrode or the shift electrode. By experience, it was found that the most suitable conditions of preparing a buried channel charge transfer device are to use a silicon wafer having a resistivity of 5 to 30 ohm-cm as a substrate, a silicon oxide film having a thickness of 0.1 to 0.2 micron as an insulating film, and to use an impurity concentration of the buried channel layer of the order of several to 30 times of that of the semiconductor substrate and a depth of the diffused impurity of about 0.5 to 2 microns. With these optimum conditions the value of (C.sub.1 +C.sub.2) becomes from one half to a small fraction of the electrostatic capacitance C.sub.O of the insulating film for each picture element. For this reason, in order to increase the quantity of the saturation light of the image sensor and to relatively increase the dynamic range it is necessary to increase the area of the picture element. As the area of the picture element increases, the area of the active region of the image sensor increases thus increasing the number of defective picture elements and decreasing the yield. Moreover, as the picture element increases, the dark current also increases.